1. Technical Field of the Invention
The present invention relates to an illumination apparatus using an LED array with a plurality of LEDs as a light source, particularly relates to an illumination apparatus capable of illuminating light having a uniform intensity to an illuminated region.
2. Description of the Related Art
There is known a liquid crystal projector for projecting image light generated by illuminating a liquid crystal panel to a screen. Although as an illumination apparatus for illuminating a liquid crystal panel, an ultra high pressure mercury lamp, a metal halide lamp or the like is used as a light source, in recent years, there is investigated to use LED capable of achieving small-sized formation and the low cost formation as a light source.
Since a brightness of individual LED is low, according to the illumination apparatus constituting a light source by LED, an LED array with a plurality of LEDs is used. However, when light from each LED is irradiated to a comparatively narrow area of an irradiated region of a display element and a total of an illuminated region is intended to cover by connecting together the narrow areas, a nonuniformity is brought about in a total irradiation region by an illumination distribution characteristic of a single piece of LED in which a center thereof is bright and a periphery thereof is dark and a dispersion in a brightness provided to individual LED.
Therefore, for example, JP-A-2003-330109, shown below, describes an illumination apparatus provided with lenses to be opposed to respective LEDs of an LED array and superposing light from respective LEDs to irradiate to an irradiated region by the lenses.
However, although the apparatus described in JP-A-2003-330109 achieves an effect of preventing a drawback by the dispersion in the brightness of each LED, the apparatus is not sufficient for preventing a nonuniformity in the brightness from being brought about to the total irradiation region. That is, light emitted from LED is provided with a directivity and therefore, the light is irradiated in a certain angular region, further, the more proximate to the center of an irradiation angle (optical axis), the higher the intensity of light. Further, since pieces of light from individual LEDs having a high center intensity in this way are superposed by the lenses and therefore, the intensity of the center is increased even in the total irradiation region.
For example, when, as shown by FIG. 6A, LEDs 2 and lenses 4 are aligned to correspond to each other by 1:1 correspondence and pieces of light from respective LEDs 2 are superposed to irradiate to the irradiated region by the corresponding lenses 4, as shown by FIG. 6B, all of the pieces of superposed light are provided with intensity peaks at centers in a lengthwise direction and a crosswise direction. Further, since the pieces of light having the high center intensities are superposed and therefore, as shown by FIG. 6C, a brightness of a center of an irradiated region 6 is increased.
Further, even when either one of a number of LEDs and a number of lenses is constituted by the other thereof multiplied by an integer, a nonuniformity in a brightness is brought about in an irradiated region. For example, when, as shown by FIG. 7A, a number of LEDs 2 is made to be four times as much as a number of the lenses 4, as shown by FIG. 7B, pieces of all of light to be superposed are provided with two intensity peaks at the same positions in a lengthwise direction and a crosswise direction. Further, the pieces of light are superposed in this way and therefore, as shown by FIG. 7C, the brightness is increased at four locations of the irradiated region 6.
Further, when, for example, as shown by FIG. 8A, the number of the lenses 4 is made to be four times as much as the number of LEDs 2, illumination light from LED 2a is split to four pieces of light having intensity peaks respectively at a right lower side in the case of a lens 4a, a left lower side in the case of a lens 4b, a right upper side in the case of a lens 4c and a left upper side in the case of the lens 4d and these pieces of light are superposed to be irradiated to the irradiated region. Similarly, illumination light from all of LEDs 2 are split to four pieces of light respectively having intensity peaks at ones of four corners and superposed to be irradiated to the irradiated region. Thereby, as shown by FIG. 8B, an intensity of illumination light is increased at four corners of the irradiated region 6.
Further, other than the above-described examples, even when, for example, the number of LEDs, and the number of lenses are provided with a common devisor other than 1 such that 6 of LEDs 2 are aligned and 4 of the lenses 4 are aligned in either one direction of a lengthwise direction and a crosswise direction as shown by FIG. 9, the nonuniformity in the brightness is brought about in the irradiated region. In the case shown by FIG. 9, pieces of illumination light from 6 of LEDs of 2e through 2j are split by 4 of the lenses 4e through 4h and thereafter superposed to be irradiated to the irradiated region. At this occasion, pieces of light irradiated from the lens 4e and the lens 4g are provided with intensity peaks at equal positions at a center and at a right side in the drawing, further, pieces of light irradiated from the lens 4f and the lens 4h are provided with intensity peaks at equal positions of a center and at a right side. Therefore, the intensity of the illumination light is increased at the center and the both left and right sides of the irradiated region.
When as in the above-described examples, an in-face intensity distribution of superposed light is patterned, the nonuniformity in the brightness is brought about in the irradiated region.